AWS Certified Solutions Architect – Professional — Question 530

A user has created a VPC with CIDR 20.0.0.0/16. The user has created one subnet with CIDR 20.0.0.0/16 in this VPC. The user is trying to create another subnet with the same VPC for CIDR 20.0.0.1/24.
What will happen in this scenario?

Answer options

• A. The VPC will modify the first subnet CIDR automatically to allow the second subnet IP range
• B. The second subnet will be created
• C. It will throw a CIDR overlaps error
• D. It is not possible to create a subnet with the same CIDR as VPC

Correct answer: C

Explanation

When creating subnets within an AWS VPC, the IP address ranges of the subnets must not overlap with one another. Since the first subnet already uses the entire VPC CIDR block of 20.0.0.0/16, any subsequent subnet creation attempt (such as 20.0.0.1/24) will fail and trigger a CIDR overlap error. AWS does not automatically resize existing subnets to make room for new ones, nor does it prevent a single subnet from utilizing the entire VPC CIDR block.